FIG. 6 is a circuit diagram showing an example of a conventional charge pump circuit. The charge pump circuit shown in FIG. 6 periodically turns on and off a plurality of charge-transfer switches 101 to 104 according to clock signals (unillustrated), and thereby charges and discharges a charge storage capacitor 105 to produce a desired output voltage Vo (2Vi) from an input voltage Vi.
The positive step-up operation mentioned above will now be described specifically. The output voltage Vo is produced in the following manner. First, during a charging period, the switches 101 and 102 are kept on, and the switches 103 and 104 are kept off. By this switching operation, the input voltage Vi is applied to one end (point A) of the capacitor 105, and a ground voltage GND is applied to the other end (point B) of the capacitor 105. Accordingly, the capacitor 105 is charged until the potential difference across the capacitor 105 becomes substantially equal to the input voltage Vi.
After the completion of the charging of the capacitor 105, during a pumping period, the switches 101 and 102 are kept off, and the switches 103 and 104 are kept on. By this switching operation, the voltage at point B is raised from the ground voltage GND to the input voltage Vi. Here, since the potential difference across the capacitor 105 is substantially equal to the input voltage Vi as a result of the previously performed charging, as the potential at point B is raised to the input voltage Vi, the potential at point A is raised to 2Vi (input voltage Vi plus charged voltage Vi). Here, point A is connected to a grounded node via the switch 104 and an output capacitor 106, and thus the output capacitor 106 is charged until the potential difference across the output capacitor 106 becomes substantially equal to 2Vi.
In this way, the charge pump circuit 100 operates through the alternating charging and pumping periods described above to turn on and off the switches 101 to 104 periodically, and thus the positively stepped up voltage 2Vi obtained by positively stepping up the input voltage Vi by a factor of two is extracted as the output voltage Vo.
As another conventional technology related to charge pump circuits, for example, in patent document 1, the applicant of the present invention discloses and proposes a charge pump circuit that varies the frequency of a clock signal based on the result of the detection of an output voltage.    Patent document 1: JP-A-2002-272091